Oh I agree, given a different time and culture, we would all be subject to the beliefs and practices of the day. Human sacrifice did not always involve ones own children, but much more likely involved conquered people. However your assertion that a parent would slit a child's throat and just move on is a bit wonky. As you have already pointed out, people then are like people now with similar emotions. Today we send our children to war, and sometimes they die. Parents don't just move on content that they did the right thing. Those people grieved just as we grieve today.
As a whole, humankind has decided that human sacrifice is unacceptable, and they have left those gods behind. Except for one of those gods. The god of the bronze-aged desert dwellers. It takes quite a bit of cognitive dissonance to say that human sacrifice is wrong, and then take part and defend a religion that is based on human sacrifice. This is not shocking. It is how religion works.
When we look at the history of the church, and see the atrocities committed in the name of a blood thirsty god, can we really argue that those people didn't think they were making their god happy? No. It was right to burn 'witches'. It was right to torture 'heretics'.
I suppose that the real question is asked by today's standards. If all those other religions were brutal and led people to do inhumane things, then why do some still cling to one of them that was just as guilty as the rest of them? So the king of Moab sacrificed his son---shocking! This newer god sacrificed his son. Amen! An entire religion was built on it.
Hopefully in years to come, more will figure out that this religion is just as brutal as other the other religions from which it evolved. In the meantime, believers will be shocked at the brutality of those dead religions, while clinging to their own brutal religion. As the OP said. Cognitive Dissonance.
NC